Orthogonal frequency division multiplexing (OFDM) has become a prevalent and widespread technique for the broadcast and transmission of signals over wireless channels and has been adopted in many wireless standards. OFDM may be united with antenna arrays at the transmitter and receiver side to improve the diversity gain and to improve the system competence on time-variant along with the frequency-selective channels, resulting in a multiple-input multiple-output (MIMO) composition.
Performance Analysis for PAPR Reduction using SLM technique in x and x Differential STBC MIMO RS OFDM Systems in Rayleigh Fading Channel 1,4 P.Sunil Kumar1, M.G.Sumithra2, M.Sarumathi3, E.Praveen Kumar4 P.G.Scholars, Department of ECE, Bannari Amman Institute of Technology, Sathyamangalam Professor, Department of ECE, Bannari Amman Institute of Technology, Sathyamangalam Assistant Professor, Department of ECE, Bannari Amman Institute of Technology, Sathyamangalam sunilprabhakar22@gmail.com Abstract— Orthogonal frequency division multiplexing (OFDM) has become a prevalent and widespread technique for the broadcast and transmission of signals over wireless channels and has been adopted in many wireless standards OFDM may be united with antenna arrays at the transmitter and receiver side to improve the diversity gain and to improve the system competence on time-variant along with the frequency-selective channels, resulting in a multiple-input multiple-output (MIMO) composition The space-time block coding (STBC) incorporated for OFDM systems with multiple transmit antennas is actually a type where coding is implemented in the time domain, that is, OFDM symbols To be more particular and specific, Alamouti’s code is taken into consideration, which is shown to be the most favourable block code for time domain coding and two transmit antennas The Differential Space time block codes (DSTBC) are ways of transmitting data in wireless communications and they are forms of space time code that not need to know the channel impairments at the receiver in order to be able to decode the signal When Reed Solomon (RS) codes are used at the demodulator side the system becomes a DSTBC RS OFDM system and a performance analysis study is investigated based on Peak to Average Power Ratio (PAPR) using Partial Transmit Sequence (PTS) for different DSTBC RS OFDM Systems under Quadrature Phase Shift Keying (QPSK) modulation scheme and its performance is evaluated in terms of Bit Error Rate (BER) under Rayleigh Multipath channel Keywords— OFDM, MIMO, STBC, SLM, Alamouti, Diversity INTRODUCTION TO MIMO-OFDM SYSTEMS Wireless channels suffer from time-varying impairments such as multipath fading, interference, and noise Diversity, such as time, frequency, space, polarization, or angle diversity, is typically used to mitigate these impairments [3] Diversity gain is achieved by receiving independent-fading replicas of the signal The multiple antenna system employs multiple antennas at either the transmitter or the receiver, and it can be either multiple-input single-output (MISO) for beam forming or transmit diversity at the transmitter, single-input multipleoutput (SIMO) for diversity combining at the receiver, or MIMO, depending on the number of transmit and receive 978-1-4673-6126-2/13/$31.00 c 2013 IEEE antennas The MISO, SIMO and MIMO channel models can be generated by using the angle-delay scattering function A MIMO system consists of multiple antennas at both the transmitter and receiver They are typically used for transmit diversity and spatial multiplexing Spatial multiplexing can maximize the system capacity by transmitting at each transmit antenna a different bit stream MISO, SIMO and MIMO can be collectively treated as MIMO, and thus the smart antenna system can be regarded as a special case of the MIMO system [4] There is always a great assurance that MIMO technology brings out a significant improvement and increase in the system capacity A MIMO system takes good advantage of the spatial diversity scheme that is obtained by spatially separated antennas in a dense multipath scattering environment In a number of different ways, MIMO systems may be successfully implemented in reality to obtain either a diversity gain to combat signal fading or to obtain either a capacity gain In a much generalized manner, MIMO technique aims to improve the power efficiency by maximizing spatial diversity Such techniques include delay diversity, space-time block codes [4], [5] and space-time trellis codes [6] According to the analysis and simulation performed in [7], MIMO can provide a spectral efficiency as high as 20-40 bits/s/Hz MIMO and OFDM is absolutely the key technique for next-generation wireless LAN and 4G mobile communications MIMO-OFDM is used in IEEE 802.11n, IEEE 802.16m, and LTE OFDM is being engaged for very keen and committed short-range communications for road side to vehicle communications and interactions and it is widely considered as a potential technique for fourth-generation (4G) mobile wireless systems OFDM generally converts a frequency-selective channel into a parallel gathering of frequency flat sub channels The subcarriers have the least amount of frequency separation which is absolutely compulsory to maintain the orthogonality so that the overlap in frequency can be easily avoided Hence, the available bandwidth is used in a very economic and resourceful manner 287 If the necessary knowledge of the channel is accessible at the transmitter, then the OFDM transmitter can familiarize or adapt its signalling scheme so that it could match easily with the channel Due to the reality of fact that OFDM uses a significant and large collection of narrowly spaced sub channels, these adaptive strategies can come within the reach of the ideal water pouring capacity of a frequency-selective channel In practice this is accomplished by using adaptive bit loading techniques, where altered or different sized signal constellations are transmitted on the subcarriers ALAMOUTI SPACE TIME CODES The idea of space-frequency coding was bought into existence in [1] The main concept behind the Spacefrequency coding is that it broadens the conjecture of spacetime coding incorporated for specific narrowband flat fading channels to broadband frequency-selective channels which are entirely time variant The most conventional and relevant application of such classical space-time coding procedures for Àat fading channels which are narrowband to OFDM seems promising and straightforward, since each and every individual subcarrier can be analysed as an independently Àat fading channel Later in [1], it was proved that the intended criteria for space-frequency codes which are operated in the space-time and frequency domain are completely different from those which are involved with the classical space-time codes for narrowband fading channels as introduced in [2] In this particular paper, the utility of the Alamouti spacetime block codes[3], which results in a space-time block coded OFDM systems with a generic two transmit antennas and one or two receive antennas is discussed and the Peak to Average Power Ratio is reduced and its performance is evaluated in terms of Bit Error Rate Generally when operating in frequency selective fading channels, the common use of conventional decoding algorithms always result in a signi¿cant performance decrease [9] This is due to the fact that the equivalent and corresponding channel matrix is no longer orthogonal Subsequently when the two transmitted symbols are independently decoded as done mostly in basic and conventional decoding algorithms cannot be appropriate anymore So for demodulation purposes Reed-Solomon Codes can be used here In this particular paper, the utility of the Alamouti space-time block codes[3], which results in a spacetime block coded OFDM systems with a generic two transmit antennas and one or two receive antennas is discussed and the Peak to Average Power Ratio is reduced and its performance is evaluated in terms of Bit Error Rate 2.1 SYSTEM MODEL An ordinary communication system incorporating the differential space-time block coding technique with just two transmit antennas and one or more receive antennas is considered for the analysis The information blocks of symbols in the transmitter side are passed to the next unit called space-time block encoder, where two symbols are embedded in each block The code words of length M = is generated by the differential space-time block encoder where 288 M signifies to the total number of transmit antennas The OFDM Modulator and the radio frequency (RF) front-ends obtains these code words and then it modulates the useful information onto the carrier frequency On the constructed receiver side, up to N receiver antennas can be efficiently made use of for reception probably The RF signals are completely down-converted and digitized in the RF front-ends and then finally passed to the OFDM demodulator unit and then the differential space-time block decoder unit [8][9] The interpretation of the received signals is done by the space-time block decoder and after that the received signals are obtained and generated for estimates as the transmitted information symbols, which are again provided simultaneously in blocks of two symbols Fig Block Diagram for Transmitter side in Differential Space time Block Coded MIMO-OFDM System Fig Block Diagram for Receiver side in Differential Space Time Block Coded MIMO-OFDM System PAPR REDUCTION TECHNIQUE USING SLM SCHEME The following algorithm steps are followed completely in the paper Step 1: The program is started and then the input bits are generated randomly Step 2: The serial data is converted into parallel data and then the sparse H matrix for Differential Space Time Encoder is calculated The Differential Space time codes are ways of transmitting data in wireless communications They are forms 2013 International Conference on Green Computing, Communication and Conservation of Energy (ICGCE) of space time code that not need to know the channel impairments at the receiver in order to be able to decode the signal by other transmit antennas For the ith antenna to jth receive antenna, each transmitted symbol gets multiplied by a randomly varying complex number As the channel under consideration is a Rayleigh channel, the real and imaginary parts are Gaussian distributed having a particular mean and variance The channel experienced between each transmitter to the receive antenna is independent and randomly varying in time The random binary sequence of +1’s and -1’s are generated Then it is grouped into pairs of symbols and then the symbols are sent in one time slot The symbols are multiplied with the channel and then white Gaussian noise is added The minimum among the four possible transmit symbol combinations is found out Based on the minimum the estimate of the transmit symbol is chosen It is then repeated for multiple values of Eb/No and then the simulation and theoretical results are plotted Table 1: Simulation Parameters for PAPR Reduction Modulation used MIMO System analyzed Fig Block Diagram for incorporating the PAPR reduction technique using SLM scheme in DSTBC MIMO-OFDM system Step 3: The input signals is then multiplied with different phase sequences and kept ready for the computation of the IFFT operation [1] Step 4: Meanwhile, the sparse matrix H is shifted and then the input bits are encoded Step 5: The input signals are modulated using QPSK modulation Step 6: The mapped sequences are computed using Inverse Fast Fourier Transforms Step 7: The signal with the lowest Peak to Average Power Ratio is selected and then proceeded to the next step Step 6: Then the parallel data is then converted into serial bits and then the PAPR value is calculated Step 7: The threshold value is calculated and then it is checked that whether PAPR>threshold value Step 8: As a final step the CCDF plot Vs probability of PAPR is computed and the plot is drawn Step 9: The bit error rate is also computed Step 10: A Performance Comparison is drawn between the Bit Error Rate obtained for (2 x 1) and (2 x 2) DSTBC MIMOOFDM systems Step 11: Stop the program PERFORMANCE ANALYSIS OF DSTBC MIMOOFDM SYSTEMS USING RAYLEIGH CHANNELS UNDER QPSK MODULATION The channel is assumed to be a flat fading, in simple terms, it means that the multipath channel has only one tap The type of modulation engaged here is Quadrature Phase Shift Keying So, the convolution operation reduces to a simple multiplication Generally, the channel experienced by each transmit antenna is independent from the channel experienced Number of parallel channels to transmit No of guard symbols FFT length Symbol rate No of time slots Window function HPA Model No of frames No of OFDM symbols/ frame Bandwidth Oversampling factor QPSK x MIMO-OFDM, x MIMO-OFDM 512 128 1024 250000 Blackman-Haris SSPA 10 MHz 4 Fig PAPR Reduction of STBC MIMO-OFDM using QPSK Modulation in both (2 x 1) and (2 x 2) Systems On the careful analysis of the figure 4, it is apparent that when QPSK modulation is engaged ,irrespective of the 2013 International Conference on Green Computing, Communication and Conservation of Energy (ICGCE) 289 number of receive antennas the PAPR remained the same for both the (2 x 1) and (2 x 2) DSTBC MIMO-OFDM Systems From the Figure 4, when QPSK is employed and for U=2, U=4, U=8, and U=16, when the SLM is incorporated in DSTBC MIMO-OFDM System with side information scheme, the PAPR remained the same for both the modulation schemes and it provided a PAPR reduction of 1.4dB, 2.5dB, 3.15dB and 3.8dB at CCDF=10-4 respectively [3] Xiaodong Zhu et al, “Simplified Approach to Optimized Iterative Clipping and Filtering for PAPR Reduction of OFDM Signals”, IEEE Transactions on Communications, Vol.61, No.5, May 2013 [4] H.Bolcksei and A.J.Paulraj, “Space-Frequency Coded Broadband OFDM Systems”, Proc IEEE Wireless Commun And Netw Conf (WCNC), Sept.2000 [5] V.Tarokh, N.Seshadri, and A.R.Calderbank, “SpaceTime Codes for High Data Rate Wireless Communication: Performance Criterion and Code Construction”, IEEE Trans Inf Theory, Vol.44, No.2,pp.744-765,1998 [6] S.M.Alamouti, “A Simple Transmit Diversity Technique for Wireless Communications”, IEEE JSAC, Vol.16, pp.1451-1458, 1998 [7] G.J.Foschini, “Layered space-time architecture for wireless communication in a fading environment when using multi-element antennas,” Bell Syst Tech.J., pp.41-59, Autumn 1996 [8] S.M.Alamouti, “A simple transmit diversity technique for wireless communications,” IEEE J.Select Areas Commun, vol.16, no.8, pp.1451-1458, Oct.1998 [9] V.Tarokh, N.Seshadri, and A.R.Calderbank, “Spacetime codes for high data rate wireless communication,” IEEE Trans.Inform Theory, vol.44, no.2, pp.744-765, March.1998 Fig Performance Analysis of (2 x 1) and (2 x 2) DSTBC MIMO OFDM Systems under QPSK Modulation On the careful analysis of Figure 5, it is evident that when QPSK modulation is engaged ,the Bit Error Rate produced is high in (2 x1) DSTBC MIMO-OFDM system when compared to that of (2 x 2) DSTBC MIMO-OFDM System CONCLUSION This paper gives a short introduction to MIMO–OFDM Systems followed by a short survey on the Alamouti Space Time Codes Further the performance analysis of PAPR and the Bit Error Rate using the Selected Mapping Technique for different Differential Space Time Block Coded OFDM is analysed for two transmit antennas and one or more receive antennas and the performance is evaluated Future works may incorporate the use of a different oversampling rate, modification of the selected mapping scheme, usage of different modulation schemes under different channel conditions to produce a low Bit Error Rate and Peak to Average Power Ratio REFERENCES [1] Tao Jiang, Chunxing Ni, and Lili Guan, “A Novel Phase Offset SLM Scheme for PAPR Reduction in Alamouti MIMO-OFDM Systems Without Side Information, “ IEEE Signal Processing Letters, Vol 20, No 4, April 2013 [2] Seyran Khademi et al, “ Constant Modulus Algorithm for Peak-to-Average Power Ratio (PAPR) Reduction in MIMO OFDM”, IEEE Signal Processing Letters, Vol.20, No.5, May 2013 290 2013 International Conference on Green Computing, Communication and Conservation of Energy (ICGCE) ... Bandwidth Oversampling factor QPSK x MIMO- OFDM, x MIMO- OFDM 5 12 12 8 1 024 25 0000 Blackman-Haris SSPA 10 MHz 4 Fig PAPR Reduction of STBC MIMO- OFDM using QPSK Modulation in both (2 x 1) and (2 x. .. DSTBC MIMOOFDM systems Step 11 : Stop the program PERFORMANCE ANALYSIS OF DSTBC MIMOOFDM SYSTEMS USING RAYLEIGH CHANNELS UNDER QPSK MODULATION The channel is assumed to be a flat fading, in simple... (ICGCE) 28 9 number of receive antennas the PAPR remained the same for both the (2 x 1) and (2 x 2) DSTBC MIMO- OFDM Systems From the Figure 4, when QPSK is employed and for U =2, U=4, U=8, and U =16 ,